The present disclosure relates in general to tunable ultrafast lasers, and more specifically, to fast, automatic wavelength tuning of mode-locked ultrafast lasers while minimizing changes in pulse characteristics thereof.
Mode-locked ultrafast lasers employing a solid-state gain medium such as Ti:sapphire can be tuned over a relatively wide range of wavelengths, from about 700 nanometers (nm) to about 1000 nm. The broad wavelength tuning range and the mode-locked, ultra-short pulses delivered by such lasers find application in spectroscopy, biology, and other scientific research and commercial applications.
Tuning of an ultrafast Ti:sapphire lasers to the full wavelength range has historically presented a number of technical challenges. For example, adjustable wavelength selective devices, such as birefringent filters (BRFs), have been introduced into the laser cavity or optical path. However, the interplay of intra-cavity peak power, gain variation in different wavelength ranges, and intra-cavity dispersion that affect the laser pulse characteristics result in complex, multiple-element laser wavelength tuning. Among the elements that must be simultaneously controlled to provide wavelength selection are adjustable wavelength selective device control, pump power control, etc. This complex and dynamic control problem has hindered the wider use of wavelength adjustable ultrafast lasers.
Various designs have been proposed to provide improved wavelength tuning of a mode-locked ultrafast laser. One example of such a proposal is illustrated in FIGS. 6 and 7, which schematically depict a tunable mode-locked laser resonator 20. With reference to FIG. 6, laser resonator 20 comprises a dispersion compensation portion 10 formed between a partially transmitting, output coupling mirror 11 and a maximum reflecting mirror 18. Fold mirrors 25 and 26 serve to compact the physical size of laser resonator 20, while still allowing a relatively long optical path length in the resonator, for example, an optical path length of over 100 cm or greater between terminating mirrors 11 and 18 of the resonator.
The wavelength tuning method in the laser resonator is illustrated with reference to the apparatus shown in FIG. 7. When the non-dispersed laser beam 22 passes the first prism 13, wavelength dependent divergence occurs, separating the wavelength content of the resonating beam 22 to the short wavelength 15 on one side and the long wavelength 16 on the other side. The wavelength selecting stop 17 is placed in the laser beam dispersed region after the second prism 14 to allow a selected wavelength, shown by beam 27 for example, to oscillate (i.e., pass through stop 17, reflect off mirror 18 and back through stop 17, through prism 14 then 13, and so on), so as to achieve wavelength tuning. To achieve proper wavelength tuning with well-balanced pulse width from the laser, the second prism 14 has to be translated along the direction indicated by arrow 19. Meanwhile, the position of stop 17 must also be adjusted, along the direction of arrow 24, to ensure proper dispersion for the oscillating wavelength. To complicate the matter further, the width of stop 17 must also be adjusted when the wavelength is varied across the tuning range. The complexity in wavelength tuning with this conventional configuration represents significant disadvantage for ease of manufacturing of the laser and user experience.
Development of a single-element control in place of the multi-element approach has enabled improved control and ease of use in wavelength selection. An example of such a single-element control is disclosed in U.S. Pat. No. 6,594,301, incorporated herein by reference, and generally illustrated in FIG. 8. The laser apparatus is arranged similar to laser resonator 20 of FIG. 6. When the non-dispersed part of the laser beam in the resonator 10 passes through first prism 13 toward second prism 14, wavelength dependent divergence occurs, separating the spectral content of the resonating beam to the short wavelength 15 on one side and the long wavelength 16 on the other side. The wavelength selecting stop 17 is placed in the laser beam dispersed region after second prism 14 in a fixed positional relationship with the second prism. When the second prism and the stop move together along the translation direction 29, a selected wavelength, shown by beam 28 for example, is allowed to oscillate, so as to achieve the wavelength tuning.
The advantage in comparison to the prior approach in FIGS. 6 and 7 is that the control of tuning is reduced to a single-element adjustment—the translation in the direction of arrow 29. Therefore, the manufacturing and use complexity is significantly reduced.
However, one shortcoming of even single-element control is the slow speed of wavelength tuning. For many applications, such as the dynamic studies of some live tissues, the speed of wavelength tuning remains a fundamental problem because the time it takes (tens of seconds) to tune from one wavelength to the other is not fast enough to resolve the physical phenomena of interest.
The slow tuning speed is a result of the inertia and momentum of the motion control subsystem. In the aforementioned U.S. Pat. No. 6,594,301, for example, the motion control subsystem (prism and stop positioning system) includes relatively large, heavy elements such as one or more prisms, stops, and a carriage to support these elements in a precise position relative to one another. Adjustment of wavelength requires precisely and rapidly moving this assembly. The inertia of the mass, and momentum while the subsystem is moving, prevents simultaneous fast motion and precise positioning for wavelength selection at high speed. Neither speed nor accuracy is optimized. Thus, there is a need for an ultrafast laser wavelength tuning mechanism in which not only a simple scheme is applied suitable for automatically controlled operation, but also a faster mechanism to enable high speed tuning of the laser wavelength.